The present invention relates to the field of dentistry, particularly the dental implant field. This invention is important because since two-stage dental implants were introduced to the U.S. market in 1982, their use and popularity have soared. Implants of this type are described in many issued patents, including inter alia, U.S. Pat. Nos. 5,376,004; 5,366,374; 5,350,300; 5,344,457; 5,342,199; 5,328,371; 5,316,476; 5,312,256; 5,312,255; 5,312,253; 5,269,686, the disclosures of which are hereby incorporated herein by reference.
Other patents and publications describing known implant systems include the following; Schulte, U.S. Pat. No. 4,486,178; Scantlebury et al., U.S. Pat. No. 4,531,916; Tatum, Jr., U.S. Pat. No. 4,531,915; Sandhaus, U.S. Pat. No. 4,466,796; Small, U.S. Pat. No. 4.439,152; Niznick, U.S. Pat. No. 4,431,416; Mozsary et al., U.S. Pat. No. 4,416,629; Branemark et al., U.S. Pat. No. 4,330,891; Branemark et al., U.S. Pat. No. 4,065,817; Cohen et al., U.S. Pat. No. 3,905,109; Stevens et al., U.S. Pat. No. 3,579,831; Predecki et al., J. Biotaed. Mater. Res., 6:401 (1972); Williams, J. Med. Eng. Tech., 266 (1977); Beder et al., O.S.,O.M. & O.P., 787 (1959); Branemark et al., "Osseointegrated Implants in the Treatment of the Edentulous Jaw", Scan. J. Plas. Recon. Surg., Vol. 11, Suppl. 16, (1977).
The most popular types of dental implant restorations are held in place by screws that thread into the implant or implant extension (abutment). One problem that has been discussed in dental meetings and in the scientific literature is the problem of screws that loosen up over time. Patients run the risk of having their prostheses and even implants destroyed because of this problem.
Pre-machined components (cylinders) have been developed by the manufacturers of dental implants to be incorporated into the restoration fabricated by the dental laboratory to provide a precision interface between the implant or implant extension and the seat of the head of the screw.
During the normal process of fabrication employed by dental laboratories, the surface of the interface against which the head of the screw fits, becomes less precise because of the investing and casting procedures.
During investing or mold-making, small minute air bubbles may get trapped in the cylinder. These small voids may be replaced by molten metal during the casting process. Expansion of the investment may not be identical to the expansion of the precision cylinder which is expanding in the mold. This might allow a small gap within the mold which would enable molten metal to flow into, eliminating the precision surface. Other laboratory processes such as cleaning out the investment after casting, as well as finishing techniques and sandblasting can ruin the precision surface. It has been suggested in the scientific literature that the heat involved in the casting process is enough to change the surface and shape of the pre-machined cylinder.
These pre-machined components are relatively expensive and even when the laboratory notices a problem with the screw seat, it is likely to try to grind away the imperfections making the machined surface equally imprecise, but less perceptible.
These surfaces against which the screw fits are often extremely difficult to inspect. They are deep within the restoration where they are not readily seen. These precision surfaces are never-the-less extremely important in obtaining the proper preload tension of the screw. Making the surface of the restoration where the head of the screw seats perfectly flat and perpendicular to the long axis of the implant will put less stress on the screw and allow for more optimum preload. This will extend the life of the screw and it will effectively solve many problems of screw-loosening.
A need was seen by the present inventor to develop a tool together with a technique for using that tool, that any laboratory could use with any type of dental implant system, to precisely mill the seat for the screw that holds the dental prostheses to the dental implant (or implant extension) to eliminate inaccuracies that occur in everyday fabrication processes in the dental laboratory.
Another important aspect of milling the screw seat pursuant to the teachings of this invention is to mill the opening through the center of the cylinder so the shaft of the screw does not contact as it passes through the restoration. If the shaft of the screw achieved contact on the axial walls of either the head of the screw or the shaft, the screw would come under tension reducing the amount of optimum preload required to keep the screw tight.
For these reasons, there is a need to mill the sides of the restoration at and above the screw seat as well as the opening beneath the screw seat. This allows the screw to be tightened so the head of the screw contacts only the base of the restoration and not the sides of the restoration, and allows the shaft of the screw to pass through the restoration without adverse contact.
One manufacturer (Interpore) saw the need for improving the interface of both the screw seat and the implant or implant extension seat after casting procedures. They produced an instrument the laboratory could use to remill these surfaces after casting. These instruments were turned by hand into the casting surface removing bubbles and debris. These instruments provided absolutely no level of precision, however, because they could cut at any angle and there was no technical means by which they could mill at the precise angle necessary to create a seat for the screw that would be perpendicular to the axis of the implant. This is important to achieve because stress is placed on the screw when the seat is not at a right angle to the shaft of the screw.
Another problem with Interpore's device is the screw seat device they used was tapered rather than flat. The vast majority of dental implants use a pan-head type screw that requires a flat surface for the head of the screw to fit against.
Implant Innovation, Inc. (31), another dental implant manufacturer, also sells and markets an instrument for milling the screw channel for UCLA abutments. This is also a manual instrument that provides no accuracy in milling the screw seat as it is operator sensitive. As with the Interpore instrument, it does not mill a flat area for the pan-head type screw and cannot be used for other types of dental implant cylinders. It is not effective in milling the screw seat so it is at the right angle to the shaft of the screw and it offers no precision surface for the screw to seat against.
Similar tools are also sold and marketed by Vident, Calcitek, Lifecore Biomedical and Attachments International with the same drawbacks and limitations.
Steri-Oss sells and markets hand instruments that mill the screw seat as well as the screw channel for a flat headed screw. This manufacturer realizes the limitations of these tools and cautions users in their prosthetic manual that over use of their tools seriously jeopardize the integrity of the screw seat. This tool is used manually and lacks the precision necessary to mill the screw seat perfectly flat and perpendicular to the long axis of the implant.
To achieve the purpose of milling the screw seat flat and at the correct 90.degree. angulation to the axis of the implant, as well as centering the tool over the implant or abutment to provide the clearance for the shaft of the screw and the head of the screw, an entire set of procedures have been developed that can be followed and used by any person with an appropriate milling instrument.